1. Field of the Invention
The present invention relates to a method for forming a bump on electrodes of a semiconductor element.
2. Prior Art.
There are two different ways in forming a bump on semiconductor elements: one is a plating method as shown in FIG. 3 and the other is a ball bonding method shown in FIG. 4.
In these methods, a semiconductor element 1 is formed with an Si layer 2 and an electrode 3 such as an Al wire formed on the Si layer 2. A protective layer 4 is formed on the electrode 3 except for the part on which the bump is formed.
In the plating method shown in FIG. 3, a barrier layer 5 is formed around the bump forming area on the electrode 3, and the bump 6 is formed on the barrier 5 by plating. The bump 6 is formed by the following process: a metal barrier is vapor deposited around the bump forming area on the semiconductor element 1, a resist is applied (as preparation for plating), plating is performed, the resist is removed, another resist is applied (for the metal barrier to be removed), the metal barrier is removed, and then the resist is removed.
One example of the ball bonding method such as the one shown in FIG. 4 is described in Japanese Patent Application Laid-Open No. 54-2662.
More specifically, as shown in FIG. 4(a), a ball is first formed on a wire 8, which passes through the capillary 7, by discharging etc. Next, as shown in FIG. 4(b), the capillary 7 is lowered so that the ball 9 is pressed down against the electrode 3 by the capillary, and then the capillary 7 is raised as shown in FIG. 4(c). In the next step, the wire 8 is clamped by a clamper (not shown) as shown in FIG. 4(d), and the clamper and capillary are raised to cut the ball 9 out of the wire, thus forming ball-shaped bump 10 on the electrode 3.
The methods above involves some problems.
According to the method shown in FIG. 3, since a considerable number of steps and expensive equipment are needed, the costs to form the bump tends to be high. In addition, the quality of the semiconductor element gradually drops as the steps progress, resulting in a poor yield.
In the ball-forming method of FIG. 4, it is necessary to form the ball 9, and the ball 9 is two or three times as large as that of the wire 8 in diameter. As a result, a narrow (bump forming) pitch is not obtainable. Also, the height of the bump 10 tends to vary depending upon the shape of the flattened ball 9 and upon how the wire 8 is cut, and this can affect the inner lead bonding which will be performed in the postprocessing.
Furthermore, the ball-bonding method usually requires the semiconductor element be heated at about 250.degree. C. so as to connect the ball to the electrode for the purpose of increasing the bondability. Thus, since the semiconductor element is heated for a long time, the color of the bump 10 changes and purple plating corrosion can result in the joint area of the bump and the electrode. In addition, a primary eutectic is made from Au and Al, which are the material for the wire and the electrode, respectively, and this has an adverse influence on the lead bonding in the postprocessing.